US 7655148 B2
A water treatment composition includes an oxidizing component and an adsorbing component, wherein one or both of the oxidizing component and adsorbing component comprise nanostructured materials. The water treatment compositions are useful in at least partially removing contaminants such as metallic or cationic arsenic, lead, chromium, and/or mercury from water.
1. A water treatment composition, comprising:
an oxidizing component; and
an adsorbing component, wherein one or both of the oxidizing component and adsorbing component comprise nanostructured materials, wherein the solid oxidizing component is an iron doped manganese oxide and wherein the water treatment composition is disposed onto a carrier suitable for water treatment or incorporated into a filtration device suitable for water treatment or disposed onto a carrier and incorporated into a filtration device suitable for water treatment.
2. The water treatment composition of
3. The water treatment composition of
4. The water treatment composition of
5. The water treatment composition of
6. The water treatment composition of
7. The water treatment composition of
8. The water treatment composition
9. The water treatment composition of
10. A water treatment composition, comprising:
a nanostructured manganese containing composition; and
a nanostructured adsorbing component, wherein the nanostructured iron-doped manganese oxide composition and the nanostructured adsorbing components are agglomerated to form a particle having an average longest dimension of at least one micro meter and wherein the water treatment composition is disposed onto a carrier suitable for water treatment or incorporated into a filtration device suitable for water treatment or disposed onto a carrier and incorporated into a filtration device suitable for water treatment.
11. The water treatment composition of
12. The water treatment composition of
13. The water treatment composition of
14. The water treatment composition of
15. The water treatment composition of
16. A method for treating water, comprising
contacting water with the water treatment composition according to
at least partially removing a contaminant from the water.
17. The method of
oxidizing the contaminant; and
adsorbing the oxidized contaminant.
18. The method of
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/726,924 filed Oct. 14, 2005, which is incorporated herein by reference in its entirety.
The United States Government has certain rights to this disclosure pursuant to United States Air Force Flight Test Center Contract Nos. F04611-03-M-1007 and FA9300-04-C-0033.
The present disclosure generally relates to water treatment. More particularly, the present disclosure relates to compositions and methods for removal of contaminants from aqueous streams.
Wastewater and natural waters (e.g., surface water or groundwater) may contain a variety of dissolved inorganic substances from natural and anthropogenic sources. Regulatory limits have been set for a number of these substances in drinking water and for discharges to natural waters, for protection of public health and of environmental quality. In many locations, the regulatory limits for many of these substances are set at very low levels, e.g., about 2 to about 50 micrograms per liter (μg/L).
Current water treatment processes, such as granular iron media adsorption, modified coagulation/filtration, reverse osmosis hyperfiltration, anion exchange, activated alumina adsorption, modified lime softening, electrodialysis reversal, and oxidation/filtration, are available and cost-effective for large municipal water treatment facilities. However, as the regulated levels of these contaminants are reduced even further, many of these processes are rendered inadequate. Furthermore, improvements on these technologies or creation of new technologies are needed for small water treatment systems (e.g., those that serve communities of 25 to 10,000 people) and point-of-use or point-of-entry systems (e.g., those for use in private wells) to treat water cost-effectively.
There accordingly remains a need in the art for improved compositions and methods for removing contaminants from aqueous streams.
Disclosed herein are water treatment compositions. In one embodiment, a water treatment composition includes an oxidizing component and an adsorbing component, wherein one or both of the oxidizing component and adsorbing component comprise nanostructured materials.
In another embodiment, the water treatment composition includes a nanostructured manganese containing composition and a nanostructured adsorbing component, wherein the nanostructure manganese-containing composition and the nanostructured adsorbing component are agglomerated to form a particle having an average longest dimension of at least one micrometer.
A method for treating water includes contacting water with the water treatment composition and at least partially removing a contaminant from the water.
The above described and other features are exemplified by the following figures and detailed description.
Referring now to the figures, which are exemplary embodiments and wherein like elements are numbered alike:
Disclosed herein are water treatment compositions and methods of making and using the water treatment compositions. The water treatment compositions are useful for removing contaminants from water. Specific contaminants include metallic or cationic arsenic, lead, chromium, mercury, or a combination comprising at least one of the foregoing.
In contrast to the prior art, the water treatment compositions generally comprise both an oxidizing component and an adsorbing component. The oxidizing component and/or the adsorbing component may be nanostructured. In an exemplary embodiment, both the oxidizing component and the adsorbing component are nanostructured. The term “nanostructured”, as used herein, refers to particles having an average longest grain dimension of less than about 250 nanometers (nm). One or both of the oxidizing component and adsorbing component comprising the nanostructured materials can be agglomerated to form particles having an average longest dimension of at least one micrometer.
The oxidizing component can include a manganese-, silver-, and/or titanium-containing composition. Suitable compositions include oxides, hydroxides, or oxyhydroxides of manganese, silver, or titanium. The adsorbing component can include a titanium-, zirconium-, aluminum-, and/or iron-containing composition. Suitable compositions include oxides, hydroxides, or oxyhydroxides of titanium, zirconium, aluminum, or iron. The oxidizing component can be doped. In addition, or in the alternative, the adsorbing component can be doped. It should be noted that the oxidizing component and the adsorbing component must have different nominal compositions. That is, while the oxidizing component and the adsorbing component can have overlapping elements or constituents, the overall composition of each component must be different.
In an exemplary embodiment, the oxidizing component is a manganese oxide an iron-doped manganese oxide, or a combination comprising at least one of the foregoing; and the adsorbing component is an iron oxide, a Mn- or La-doped iron oxide, a zirconium hydroxide, a Mn- or Fe-doped zirconium hydroxide, a titanium hydroxide, a Mn- or Fe-doped titanium hydroxide, or a combination comprising at least one of the foregoing.
Desirably, the overall water treatment composition is highly porous so as to allow the treated water to flow therethrough, yet also absorb contaminants during treatment. In an exemplary embodiment, one or both of the oxidizing component and the adsorbing component has a so-called “bird's nest” fibrous structure, as shown in
The water treatment compositions can be disposed onto a carrier and/or incorporated into a filtration device.
A method for using any of the water treatment compositions disclosed herein generally includes contacting water with the water treatment composition and at least partially removing a contaminant from the water. In one embodiment, at least partially removing the contaminant can include oxidizing the contaminant and adsorbing the oxidized contaminant.
The adsorption of a contaminant onto the water treatment compositions generally occurs by ligand exchange. Advantageously, because these contaminants strongly bond to the water treatment media, the media, once exhausted, can be disposed of as a non-hazardous waste in landfills.
The disclosure is further illustrated by the following non-limiting examples.
MnO2 was synthesized by oxidizing manganese sulfate (MnSO4) with potassium permanganate (KMnO4) under acidic conditions according to reaction (1).
The reaction time and temperature were adjusted to control the crystallinity, morphology and catalytic activity of the material. A schematic illustration showing the gradual transformation of MnO2 nanoparticle agglomerates into a mass of interconnected nanofibers is shown in
Powder X-ray diffraction (XRD) patterns of the MnO2 were obtained on a Bruker D5005 or D8 diffractometer equipped with a 2.2 kilowatt (kW) Cu X-ray tubes. The equipment was run at 40 kilovolt (kv) and 20 milliAmpere (mA) by step-scanning with increments of 5 degrees per minute (°/min). XRD analysis (as shown in
Specific surface area (SSA), total pore volume, micropore volume, and mesopore size distribution of the as-synthesized MnO2 nanofibers were analyzed on a QuantaChrome NOVA4200e surface area and pore size analyzer using the BET method. SSA and pore analyses of MnO2 samples are listed in Table 1. Pores are classified by diameter as micropores (<10 Å), mesopores (10 to 250 Å) and macropores (>250 Å). The MnO2 synthesized at room temperature possessed a higher SSA than the MnO2 synthesized at 70° C., although its total pore volume was lower. This suggested that it had a smaller particle size, contained micropores, or both. The results shown in Table 1 confirmed that MnO2 synthesized at room temperature contained micropores, which contributed about 75 square meters per gram (m2/g) of its 259 m2/g SSA. Mesopore size distributions of these two materials are shown in
The MnO2 synthesized at room temperature exhibited a singular pore size distribution with a pore radius peak of about 17 Angstroms (Å). In contrast, the MnO2 synthesized at 70° C. exhibited a bimodal pore size distribution with pore radii peaks at about 20 Å and about 93 Å, respectively. The 93 Å peak was much broader than the 20 Å peak; it covered a pore radius from about 50 Å to about 300 Å. Most pores in this range were interparticle voids that formed among particles. Theoretically, both micropores and mesopores are accessible for arsenic species in the water, because the radius of arsenic ions, such as AsO4 3− with r=0.47 Å and H3AsO3 with r=0.69 Å, are much smaller than the sizes of the observed micropores and mesopores.
High resolution scanning electron microscope (HR-SEM) images were acquired at 10 kilo-electron Volts (KeV) using a Jeol 890 inlens SEM. HR-SEM images of MnO2 are shown in
It was also observed that interparticle voids formed among MnO2 spikes about 50 Å to about 300 Å. This is consistent with pore size distribution results. Optical and scanning electronic microscope images of reconstituted MnO2 particles are shown in
In this example, the MnO2 nanofibers synthesized according to Example 1 were evaluated for their efficiencies in oxidizing As(III) to As(V) in comparison with a commercially available product. The comparisons were made using rapid small scale column tests (RSSCTs) under a variety of experimental conditions. Using adsorption isotherm tests, MnO2 nanofibers were also evaluated for their performance in removing As(V) and Pb(II) from drinking water.
For all evaluation experiments, a so-called “challenge water” of the composition described in Table 2 was used. It has the same anionic composition as the National Sanitation Foundation (NSF) International Standard 53 Challenge Water used for evaluation of point-of-use devices for arsenic removal. Its calcium and magnesium concentrations were lowered to give the water greater long-term stability, as the NSF water is stable only for about 24 to about 48 hours, whereas the isotherm tests described herein were conducted for greater than or equal to about 48 hours. The following reagent grade salts were used to prepare the challenge water: NaNO3, NaHCO3, Na2HPO4 H2O, NaF, Na2SiO3·9H2O, MgSO4·7H2O, and CaCl2·2H2O. Interfering Fe2+ ions in As(III) oxidation tests were introduced from ferrous ammonium sulfate (EM Science). Mn2+ and S2− were prepared from MnSO4 H2O, and Na2S 9H2O, respectively.
Both low and high levels of dissolved oxygen (DO) were used in the challenge water during As(III) oxidation tests. Low-DO, i.e., less than about 80 parts per billion (ppb) O2, challenge water was prepared by sparging the challenge water with N2 for 1.5 hours. High-DO challenge water was prepared by sparging water with air for 15 minutes to a dissolved oxygen saturation concentration of approximately 8.3 mg/L. As(III) was spiked into the challenge water, and then freshly prepared Fe2+, Mn2+ or S2− solutions were spiked. During the experiments, the pH of the challenge water was adjusted to a range of about 6.5 to 8.5 by using dilute HCl and NaOH solutions.
For the As(III) oxidation tests, rapid small scale column tests (RSSCTs) were used to evaluate MnO2 nanofibers for their efficiency in oxidizing As(III) to As(V). The tests were conducted under a variety of experimental conditions including variable pH, empty bed contact time (EBCT), low/high dissolved oxygen (DO), and the absence or presence of interfering reductants (Fe2+, Mn2+, or S2−).
It is noted here that MnO2 nanofibers were not directly used in the column test, because of their small particle size (about 10 μm) which would have given an extremely high pressure drop in a packed bed. To achieve reasonable RSSCT flow rates, the MnO2 nanofibers were granulized by the spray-drying technique described in Example 1 to form larger, but porous particles. Granulized, porous MnO2 particles (having diameters of about 125 to about 180 μm) were loaded into a 1 centimeter inner diameter glass column with total volume of 1.0 mL. The column was then backwashed with deionized water to remove fines.
The As(III) oxidation tests were conducted by pumping the Low-DO challenge water containing As(III) through the column at a controlled flow rate. To speciate the effluent, 10-mL samples of the effluent from the RSSCT were collected and preserved with EDTA-HAc. As(III) concentration in the effluent was analyzed by hydride-generation atomic absorption spectrophotometry (HG-AAS).
For the As(V) and Pb(II) adsorption isotherm tests, a 2-day wet-slurry isotherm procedure was used for adsorption isotherm tests. First, adsorbents were screened with a set of 200 and 325 mesh sieves. Powder that was trapped between these two sieves, i.e., having a particle size between about 45 to about 75 μm, was used to make a suspension with a powder content of 1.00 g/L. The suspension was vigorously stirred to keep powders suspended. Predetermined aliquots of the vigorously stirred suspension were pipetted into 150-mL bottles to yield appropriate adsorbent concentrations for the equilibrium adsorption tests. Then, 100-mL aliquots of As(V) or Pb(II) spiked synthetic water was added to each bottle. Adsorbent media dosages that were tested were: 0, 0.25, 0.5, 1, 2, and 5 mg MnO2 in every 100 mL As(V) or Pb(II) spiked synthetic water. The dosed bottles and the blank (no adsorbent) were placed in a covered wooden box and rotated at 15 revolutions per minute (rpm) for 2 days at ambient temperature (about 23° C.) to reach adsorption equilibration. It was determined that a reasonable approach to equilibrium was reached in 48 hours. After the equilibration, a 10-mL water sample was collected from each bottle and microfiltered with a 0.2 μm filter disc to remove all suspended particles (adsorbent media). The water samples were preserved with concentrated HNO3 prior to analysis. The acid concentration in the water sample was about 1% (v/v). For As(V) adsorption isotherm tests, the challenge water was spiked with 200 μg/L As(V). Arsenic concentrations in the preserved water samples were also measured by HGAAS. The mass of arsenic adsorbed onto the media was determined from the difference in soluble arsenic concentrations in the equilibrated samples and the blank (no adsorbent). For Pb(II) adsorption isotherm tests, the challenge water was spiked with 500 μg/L Pb(II), and the Pb(II) concentrations in the preserved water samples were measured by inductively coupled plasma mass spectrometry (ICP-MS). The mass of lead adsorbed onto the media was determined from the difference in soluble lead concentrations in the equilibrated samples and the blank (no adsorbent).
The arsenic analysis was performed using a Perkin-Elmer (Model Zeeman 5000) atomic absorption spectrometer (AAS) coupled with a Perkin-Elmer FIAS-100 unit for hydride generation for the determinations of As(III) and As(total). The arsenic lamp was an electrodeless discharge lamp (EDL) operated at 8 W from an external power supply. To determine total arsenic, water samples were treated with L-cysteine in a 2 moles per liter (M) HCl solution to reduce As(V) to As(III) which was determined by the HGAAS. For the determination of As(III) in the presence of As(V), the carrier HCl solution was replaced by a 2 M citric/citrate buffer solution at a pH of about 5.0. Arsine was generated using 0.2% sodium tetrahydroborate in 0.05% NaOH. Under these condition As(V) was not converted to AsH3 and did not interfere in the determination of As(III). As(V) was calculated from the difference between As(total) and As(III). The detection limit (DL) was 0.04 μg/L for As(total) and 0.096 μg/L for As(III).
Short-term As(III) oxidation tests at high-DO were conducted to compare the efficiency of a commercially available MnO2, FILOX-R, with nanoporous MnO2 in oxidizing As(III) to As(V) at three different EBCTs (2.0, 1.0, and 0.50 minutes) and at two pHs (8.3 and 6.5). Both FILOX-R and naofibrous MnO2 granules completely oxidize As(III) to As(V). No distinguishable performance was observed under these test conditions.
In the short-term, low-DO experiments, the EBCT was further reduced to about 0.125 minutes to differentiate the oxidizing efficiencies of both media. Table 3 compares the performances of FILOX-R and granulated nanofibrous MnO2 media at different EBCTs for low-DO challenge water without any interfering reductant at pH 8.3 and 7.5. Up to about 0.25 minutes EBCT there was no significant difference in oxidizing performance between the two media. But there was a clear difference in the As(III) oxidation rate at an EBCT of 0.125 minutes. Granulated nanofibrous MnO2 media was more effective than FILOX-R for rapid oxidation of As(III) to As(V). At higher pH both media showed better performance than at lower pH; at both pHs the oxidizing efficiency of granulated nonofibrous MnO2 media was better than FILOX-R.
Based on the short-term experiments at low and high-DO, it could be concluded that DO had no significant effect on oxidizing efficiency up to EBCT 0.5 minutes for both media.
Long term experiments with interfering reductants were carried out. To study the effects of the interfering reductants, except sulfide, on the oxidation performance, a run length of more than 2100 bed volumes (BV) was considered. In the presence of sulfide, arsenic sulfides along with many water-soluble thioarsenite species are formed, and accurate differentiation between As(III) and As(V) cannot be made. The availability of the free As(III) decreased with time. So, in the presence of sulfide, only short-term experiments were conducted to observe the effects of sulfide on oxidizing efficiency of the granulated nanoporous material and FILOX-R.
In the absence of interfering reductants, the results of the longer-term oxidation performances of nanofibrous MnO2 granules and FILOX-R are shown in
As can be seen in
When 2.0 mg/L Fe(II), as an interfering reductant, was present in the feed, As(III) oxidation was reduced for a short period of time for nanofibrous MnO2 granules, and then the efficiency decreased slowly to about 1200 BV. After about 1200 BV, the oxidative ability was substantially constant. With FILOX-R, initially up to about 240 BV, Fe(II) had no effect on As(III) oxidation, but after about 240 BV, the oxidative efficiency decreased gradually up to about 2160 BV. These results, which are shown in
The effect of 0.2 mg/L Mn(II) as an interfering reductant on both media's As(III) oxidation efficiency was studied at a pH of about 7.5 and an EBCT of 0.125 minutes, and the results are shown in
When both As(III) and sulfide are present in anoxic water insoluble sulfides and soluble sulfide complexes are formed. It has previously been determined that under anoxic conditions in the presence of sulfide, As(III) forms insoluble As2S3 and soluble thioarsenite species depending on the pH of the solution and the concentrations of As(III) and S2. Therefore, in waters containing sulfide and As(III), colloidal and anionic forms of As(III) are expected, which render the current As(III)/(V) speciation methods inapplicable. Oxidation of As(III) to As(V) cannot be accurately determined in these waters using the EDTA-HAc preservation speciation method that was used for the As(III) oxidation studies in the absence of sulfide. Thus, the differential-pH hydride generation speciation method was employed to overcome the analytical problem with As(III/V) preservation-speciation in the presence of sulfide.
The oxidation efficiencies of the nanoporous MnO2 and FILOX-R were calculated at specified time intervals and compared in
The ability of MnO2 to adsorb As(V) from the challenge water was evaluated using batch isotherms tests. As(V) adsorption isotherms of MnO2 are shown in
The As(V) adsorption capacity of MnO2 is substantial, although not as high as GFH. MnO2 is a good oxidant for As(III) to As(V), but was not expected to have good As(V) adsorption capacity. The unusually high As(V) adsorption capacity that was found for the MnO2 nanofibers is believed to be related to its high surface area.
Finally, the ability of MnO2 to adsorb Pb(II) from the challenge water was evaluated using batch isotherms tests. The Pb(II) adsorption isotherm for nanofibrous MnO2 is shown in
From the experiments in this example, it was concluded that the nanofibrous MnO2 not only possessed a high efficiency in converting As(III) to As(V), but also a relatively high adsorption capacity for As(V) and Pb(II).
Fe2O3 was synthesized by preheating 100 mL water in a beaker to about 90° C. A 6M NaOH solution and 100 mL of a 2M Fe(NO3)3 solution were co-precipitated into the hot water while it was continuously heated and stirred. The pH of the precipitate was controlled at about 7.5. The precipitate was continuously refluxed at about 90° C. for 6 hours. The precipitate was then filtered, washed with water, and dried in an oven at about 100° C. The dried material was ground into a powder for further testing.
The SSA and pore analysis of this material was conducted using BET method as described in Example 1. The SSA of this material was about 154 m2/g and the total pore volume was about 0.21 cm3/g. XRD analysis revealed that this material possessed the α-Fe2O3 structure. The PZC of this material was at a pH of about 8.4.
A Mn-doped Fe2O3 was made following the process of Example 3 except that 8.73 g MnSO4·H2O was dissolved into 30 mL water and was mixed with 100 mL of the 2M Fe(NO3)3 solution before the precipitation was conducted. Doping dramatically changed the material's properties. The SSA of this material was about 259 m2/g and the total pore volume was about 0.48 cm3/g. XRD analysis revealed that this material had the structure of a ferrihydrate. The PZC of this material was at a pH of about 7.1.
A La-doped Fe2O3 was made following the process of Example 3 except that LaCl3 was dissolved in water and was mixed with 100 mL of the 2M Fe(NO3)3 solution before the precipitation was conducted. The SSA of this material was about 174 m2/g and the total pore volume was about 0.41 cm3/g. XRD analysis revealed that this material had the structure of hematite. The PZC of this material was at a pH of about 8.8.
A Fe-doped MnO2 was made following the process of Example 1, with the exception that Fe(NO3)3 was used as a precursor and was added simultaneously into a beaker containing hot water with the MnSO4 and the KMnO4. The SSA of this material was about 503 m2/g and the total pore volume was about 0.44 cm3/g. XRD analysis revealed that doping reduced the crystallinity of the MnO2 structure. The PZC of this material was at a pH greater than 3.0, which is the PZC pH for the undoped MnO2 of Example 1.
Various composites were prepared using the iron oxides and manganese oxides (both undoped and doped) described herein. The general procedure included dispersing about 1600 g of an iron oxide composition (doped or undoped) in 1 L water. Next, about 400 g of a nanofibrous manganese oxide composition (doped or undoped) were added into the iron oxide suspension and mixed using high power mechanical stirrer. About 2 weight percent (wt %) of a binder was added and mixed. The slurry was spray dried to form nanocomposite granules with a controlled particle size.
RSSCTs were carried out on a variety of samples. The first experiment studied the oxidation and adsorption efficiency of As(III) using composite Fe2O3—MnO2 (Sample No. 072105-B) media at pH 7.5. The second experiment determined the breakthrough of As(V) using Fe2O3—MnO2 (072105-B) media at pHs 6.5, 7.5, and 8.5, and compared them with the As(V) breakthrough of the commercially available benchmark, GFH. The third experiment determined the breakthrough of As(V) using Mn-doped Fe2O3 (072105-A) media at pH 7.5.
For these RSSCTs, a 1 cm inner diameter glass column containing 4 cm3 of the media (60×40 mesh), which was carefully loaded into the column, was used. Similar glass columns were prepared for GFH. A peristaltic pump was used to pump the feed solution through the columns at a flow rate of about 8 mL/min (EBCT of about 0.5 min). Arsenic-containing challenge water was analyzed for total arsenic or As(III) in the effluent and feed solutions. All samples were preserved with concentrated HNO3 (1 mL/L) or EDTA-acetic acid (1.34 mM EDTA and 87 mM acetic acid) when speciated. RSSCTs were conducted at different pHs using As(V) and As(III). Table 4 presents the experimental conditions for the RSSCTs.
RSSCTs were conducted to study the oxidation of As(III) and adsorption of arsenic using the composite Fe2O3—MnO2 (072105-B) media at a pH of about 7.5 at an EBCT of about 0.5 minutes. The arsenic breakthrough curves of the RSSCTs are shown in
Next, RSSCTs were conducted using composite Fe2O3—MnO2 (072105-B) media and GFH at three different pHs at an EBCT of 0.5 min (Experiments 2-7 in Table 4). Fifty parts per billion (μg/L) As(V) was used in all experiments to evaluate the effectiveness of the composite media and the GFH. The As(V) adsorption isotherms for the composite Fe2O3—MnO2 (072105-B) media at pHs of about 6.5, about 7.5, and about 8.5 are shown in
It has been documented that As(V) is more efficiently removed than As(III). Thus, it is important to compare the arsenic removal efficiency of the composite Fe2O3—MnO2 (072105-B) media for a pure As(V) feed compared with a pure As(III) feed.
The Fe-based media produced in Examples 3 and 4 were also used to study the removal efficiency of arsenic. RSSCTs were conducted using an Fe-based media (072105-A) at a pH of about 7.5 at an EBCT of about 0.5 minutes and compared with the composite Fe2O3—MnO2 media (070521-B) and GFH. The results are shown in
Finally, Pb(II) absorption isotherms and breakthrough curves were obtained for the composite Fe2O3—MnO2 media. 500 μg/L Pb2+ spiked NSF 53 challenge water was used. The 2-day isotherms at pHs of about 6.5, about 7.5, and about 8.5 are shown in
Zirconium hydroxide, titanium hydroxide, and their doped forms were synthesized via co-precipitation of appropriate salt precursors with caustic solution similar to what was described above in Examples 1, 3 and 4. The salts used for the synthesis of zirconium hydroxide and titanium hydroxide were ZrOCl2 and TiOSO4, respectively. Dopants were introduced from corresponding salt precursors that were homogeneously mixed with ZrOCl2 or TiOSO4 solutions. To achieve steady and controllable reaction conditions, the precursors were added simultaneously into vigorously stirred water. The flow rates of both solutions were adjusted so that the pH of the produced hydroxide slurry was at a target value throughout the reaction. The hydroxides were then filtered and washed thoroughly with deionized water to remove unwanted byproducts, and finally dried overnight in an oven at 120° C.
The SSA and total pore volume data for zirconium hydroxide, titanium hydroxide, and their doped forms are listed in Table 5. Doped hydroxides exhibited a significantly higher surface area than their undoped counterparts. Titanium-based media exhibited a significantly greater SSA than the zirconium-based media. The total pore volume of titanium-based materials was also higher than that of zirconium-based materials, which suggested that titanium-based media were more porous than zirconium-based media.
The data revealed that undoped Ti(OH)4 possesses a considerable amount of micropores (greater than about 60%), which contribute approximately 78% of its total surface area. Doping with manganese significantly increased total pore volume of Ti(OH)4, while the micropore volume decreased slightly from about 0.12 cm3/g to about 0.10 cm3/g. This suggested that the doping created a significant amount of mesopores, resulting in a higher SSA. Compared to Ti(OH)4, Zr(OH)4 is less porous; but, like Ti(OH)4, the porosity is significantly increased when doped. To summarize, doping has shown to be effective in increasing pore volume of these materials.
The PZCs of zirconium hydroxide and titanium hydroxide powders were found to be at pHs of about 8.08 and about 8.15, respectively. Doping with Mn slightly increased the PZCs of these materials to pHs of about 8.50 and about 8.24, respectively. When these materials were in contact with aqueous solutions that had pHs lower than their PZC, the surfaces of these materials developed a positive charge and encouraged the adsorption of negatively charged species in water, such as H2AsO4 −, HAsO4 2−, and AsO4 3−. The pH of drinking water is around 7.5. Thus, these two materials are expected to be able to quickly adsorb H2AsO4 −, and HAsO4 2− from drinking water.
XRD analysis on these samples revealed that the as-synthesized titanium hydroxide is amorphous, but transforms into the anatase structure type of TiO2 after a 600° C. heat treatment for about 2 hours. The powder XRD patterns are shown in
SEM images of undoped zirconium hydroxide, Mn-doped zirconium hydroxide, and Mn-doped titanium hydroxide are shown in
In this example, adsorption isotherm tests were used as screening tests to identify the titanium-based and zirconium-based media that possessed the highest As(V) adsorption capacity from each group. The selected media were further evaluated for As(V) adsorption performance under various pHs and influence of competing ions. Finally, rapid small scale column tests were conducted to compare As(V) breakthrough of the selected media and compared with a commercially available benchmark iron-based adsorbent, Granular Ferric Oxide, or GFO.
The particle size of the as-synthesized powders was about 10 micrometers in diameter, which was not directly applicable for column tests. The nanoscale powders were reconstituted into porous spherical particles with an average particle size of approximately 200 micrometers in diameter.
Six different materials, including nanoporous Zr(OH)4(s) and Ti(OH)4(s) and their doped forms, were screened using two-day isotherm tests in 200 μg/L As(V) spiked NSF Challenge Water at pH 7.5, as described above in Examples 2 and 5. Dosages of materials in the 2-day isotherms tests and the results are listed in Table 6 and 7 for the zirconium samples and the titanium samples, respectively.
From the As(V) adsorption isotherms of zirconium-based media, it was clear that the Mn-doped zirconium sample possessed the highest adsorption capacity, followed by the Fe-doped media. Undoped media had the lowest adsorption capacities. Like their zirconium counterparts, the Mn-doped titanium-based media outperformed Fe-doped and undoped titanium-based media. Thus, Mn-doped Zr-based and Mn-doped Ti-based media were selected for further evaluation under pH variations and the influence of competing ions.
The ability of these media to adsorb arsenic (V) from the challenge water was evaluated using batch isotherms tests. After developing the isotherms, the data were fitted to the Freundlich equation (2), shown above. The As(V) adsorption isotherm experiments were carried out using common background ions in the solution at three different pHs.
Batch isotherm tests with the NSF challenge water were conducted on the Mn-doped Zr- and Ti-based media at different pH's, with varying concentrations of competing ions including silica (5 and 20 mg/L) and phosphate (0 and 40 μg/L).
First, two-day adsorption isotherms were performed with the challenge water containing 40 μg/L of phosphate (P). These results were compared with challenge water without phosphate (P=0 μg/L). For the GFO, phosphate did not have a significant impact on the As(V) adsorption capacity of GFO. Though slightly better adsorption capacities were obtained in the absence of phosphate, it was not significant enough to say that, in the presence of silicate and other background ions, phosphate ions compete with the As(V) for adsorption sites at the tested concentration. The Mn-doped Zr- and Ti-based media followed similar trends. That is, there was no significant difference in absorption between the presence and absence of phosphate in the challenge water containing silicate and other background ions. The summary of the experimental results are presented in Table 8, from which it was concluded that the presence of phosphate did not reduce the adsorption capacities of the adsorbents.
Next, experiments were performed using the challenge water with the standard concentration of silica (about 20.0 mg/L) and with low silica (about 5.0 mg/L) concentrations to study the effect of silica on adsorption capacities of the GFO, Zr-based and Ti-based media at pHs from about 6.5 to about 8.5. The data is summarized in Table 9. Each of the materials performed better (i.e., adsorbed more As(V)) at the lower concentration of silica. At the higher concentration of silica, the silica competed with As(V) ions for sites on the adsorbents. At a pH of about 6.5 and a silica concentration of about 5.0 mg/L, the Zr-based media exhibited a slightly higher As(V) adsorption capacity than the GFO. The presence of silica greatly reduced the adsorption capacity of the Zr-based material. Under all other conditions, the As(V) adsorption capacity of the Zr-based media was much greater than that of Ti-based media.
RSSCTs were carried out to determine the relative breakthroughs of As(V) for the Zr-based media and the benchmark media, GFO at a pH of about 7.5. The RSSCTs were conducted as described above in Examples 2 and 5. As(V) breakthrough curves for the GFO and the Zr-based media are shown in
While the disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.
Also, as used herein, the terms “first”, “second”, and the like do not denote any order or importance, but rather are used to distinguish one element from another, and the terms “the”, “a”, and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. Furthermore, all ranges directed to the same quantity of a given component or measurement are inclusive of the endpoints and independently combinable.